Certain applications require the availability of an analog signal for trimming electronic systems, e.g., when changes in an electrical quantity are to be adjusted for any possible variations in ambient and/or processing conditions. In particular, trimming during an EEPROM erasing operation, as described in an article "An improved method for programming a word-erasable EEPROM", G. Torelli and P. Lupi, ALTA FREQUENZA, Vol. LII, No. 6, November/December 1983, pages 487-494, involves the driving of high capacitive impedance inputs. For such cases, a circuit is needed which can provide for both coarse and fine adjustments of the trimming signal so that the time requirements of the overall control action can be kept low.
The terms fine/coarse adjustment mean here a control form which can produce minimum and maximum changes in the control parameter. Thus, the trimming pulses are amplitude, rather than time, modulated.
A prior approach to the provision of a stepped waveform voltage is based on the digitally controlled resistive partition principle as described by Tedrow et al. in U.S. patent application No. PCT/US95/05588 entitled "High precision voltage regulation circuit for programming multilevel flash memory". However, the voltage quantization in this reference is tied to the number of passive components (resistors), since fine change ranges can only be obtained by using a large number of resistors.
Many applications require that the staircase in the trimming voltage waveform have a suitably steep rising edge, e.g., where the staircase derivative is slight, so that the loss in useful time can be a small percent. In this case, the equivalent output resistance of the network that is generating the voltage staircase should be quite low, if the capacitive load is a large one.
What is needed is a circuit for generating a voltage having a stepped waveform whose step can be programmed in digital form, which is more accurate and economical than known circuits, and can drive high input capacitance nodes.